1. Field of the Invention
This invention relates to a method and apparatus for bonding orthodontic appliances such as brackets to a patient's teeth. More specifically, the present invention relates to a bonding method for orthodontic appliances wherein precise positioning of each appliance on the patient's teeth is carried out in part by use of a transfer tray and fixture components connected to the transfer tray.
2. Description of the Related Art
Orthodontic treatment involves movement of malpositioned teeth to orthodontically correct locations. During treatment, tiny appliances known as brackets are often fixed to the patient's anterior, cuspid and bicuspid teeth, and an archwire is placed in a slot of each bracket. The archwire forms a track to guide movement of the teeth to desired locations. Ends of the archwire are often connected to buccal tubes that are in turn secured to the patient's molar teeth. The brackets, buccal tubes and archwires are commonly referred to as "braces".
One type of orthodontic treatment technique is called the "level-arch" technique, and involves placing the brackets on the patient's teeth at certain selected locations so that the "U"-shaped archwire extends in a generally level plane at the conclusion of treatment. When the archwire is initially installed on the brackets, the malpositioned teeth may cause the wire to deviate from its normally planar configuration (in horizontal view) and from its normally smoothly curved configuration (in plan view). However, the inherent resiliency of the archwire tends to urge the brackets and hence the associated teeth toward a level array wherein the archwire re-assumes its normally planar and smoothly curved configuration. The level-arch technique is considered satisfactory by many orthodontists because the need for bends, steps or other adjustments in the archwire is reduced and in many cases eliminated, resulting in a savings of time for both the orthodontist as well as the patient.
As can be appreciated, the degree of success of the level-arch technique is related in part to the position and orientation of the brackets and buccal tubes on the patient's teeth. For example, if one of the brackets is bonded to a patient's tooth at a location that is too close to the patient's gingiva (i.e., the patient's gums) relative to the placement of brackets on adjoining teeth, that tooth will protrude outwardly an excessive distance in an occlusal direction (i.e., in a direction toward the outer tips of the patient's teeth) relative to adjoining teeth at the conclusion of treatment if all of the brackets are aligned in a level array. In such an instance, the orthodontist can correct the orientation of the malpositioned tooth by placing bends or steps in the archwire at locations adjacent each side of its bracket, but such a practice entails additional work for the orthodontist and may also increase the overall length of treatment time.
As a consequence, many suggestions have been made in the past for improving the placement accuracy of orthodontic appliances during the procedure of bonding the appliances to the patient's teeth. For example, height gauges such as the well-known "Boone" gauge provide a means for indicating a desired position of the appliance on a tooth relative to the occlusal edge of the associated tooth. The Boone gauge has a flat surface that is placed over the occlusal edge of the tooth, and a fixed pin spaced from the flat surface scribes a mark to indicate the desired occlusal-gingival position of the archwire slot of the appliance on the tooth. Once the mark has been scribed, a small quantity of adhesive is placed or "buttered" on the base of the appliance and a tweezers or other hand instrument is used to place the appliance on the chosen tooth. Next, the appliance is shifted as may be necessary to bring the appliance to the selected position.
A somewhat similar device is known as a positioning jig, and is supplied by the manufacturer of the appliance as a component to be disposed of once the appliance has been properly positioned on the selected tooth. An example of a positioning jig for use with an orthodontic bracket is described in U.S. Pat. No. 5,429,229, and is made of a flexible plastic material having stirrups that are received over occlusal and gingival edges of tiewings of the bracket. The jig described in U.S. Pat. No. 5,429,229 can be grasped by a suitable hand instrument and used to support the bracket during placement and bonding of the bracket to the tooth. The jig includes stepped portions that are intended for alignment with the occlusal edge of the selected tooth. Once the adhesive has hardened and the bracket is securely bonded to the tooth, the positioning jig is released from the bracket by bending the jig until the stirrups disengage the tiewings.
Other types of appliance positioning devices are described, for example, in U.S. Pat. Nos. 4,455,137 and 4,850,864. In the device of U.S. Pat. No. 4,850,864, the appliance is grasped by opposed jaws, and the device also includes a gauge for placing the appliance on a tooth surface at a location that is a pre-determined distance from the occlusal edge of the tooth. Another example of a positioning device is known as the "Dougherty" gauge, which has a blade that is received in the archwire slot of the appliance to support the appliance during bonding. The Dougherty gauge also has an arm that is spaced a fixed distance from the blade, and the arm is placed in contact with the occlusal edge of the tooth during bonding so that as a consequence the appliance is positioned a pre-determined distance from the tooth's occlusal edge.
While the bonding techniques described above are considered satisfactory by some practitioners, there are shortcomings that are inherent with such techniques. For example, access to the surfaces of certain malpositioned teeth (such as the bicuspid and molar teeth) may be difficult. In some instances, and particularly in connection with posterior teeth, the practitioner may have difficulty seeing the precise position of the bracket relative to the tooth surface. Another problem with the above described techniques concerns the significant length of time needed to carry out the procedure of positioning and bonding a bracket to each individual tooth, which is a nuisance both to the patient as well as to the orthodontist. The risk of moisture contamination from the patient's saliva also increases as the time increases that the patient is awaiting completion of the bonding procedure. The above factors may also unduly impair the accuracy of placement of the brackets on the teeth and/or increase the chance that the ultimate adhesive bond will not have sufficient strength to retain the brackets on the teeth during the course of orthodontic treatment.
Bonding techniques known as "indirect bonding" avoid many of the problems noted above. In general, indirect bonding techniques involve the use of a transfer tray having a shape that matches the configuration of at least part of one of the patient's dental arches. A set of brackets is releasably connected to the tray at certain, pre-determined locations. Once adhesive is applied to the base of each bracket, the tray is placed over the patient's teeth until such time as the adhesive hardens. Next, the tray is detached from the teeth as well as from the brackets, often with the result that all of the brackets that were previously connected to the tray are now bonded to their respective teeth at certain intended, pre-determined locations. The procedure is often duplicated for the patient's other dental arch.
In more detail, one known method of indirect bonding includes the steps of taking an impression of the patient's dental arch and then making a replica plaster or "stone" model from the impression. A sealing solution (such as Liquid Foil brand sealing solution from 3M) is applied to the stone model and allowed to dry. If desired, the teeth of the model are marked with a pencil to assist in placing the brackets in ideal positions.
Next, the brackets are temporarily bonded to the sealed stone model. Optionally, the bonding adhesive can be a chemical curing adhesive (such as Concise brand from 3M) or a light curable adhesive (such as Transbond XT or Transbond LR adhesive from 3M). Optionally, the brackets may be adhesive pre-coated brackets such as described in U.S. Pat. Nos. 5,015,180, 5,172,809, 5,354,199 or 5,429,299.
A transfer tray is then made by placing matrix material over the model as well as over the brackets on the model. For example, a plastic sheet matrix material may be placed over the model and brackets and then heated in an oven. A vacuum source is used to evacuate air between the matrix material and the model. As the matrix material is heated, the plastic sheet material is drawn down over the model and assumes a configuration that precisely matches the shape of the replica teeth of the stone model and adjacent brackets.
The plastic model is then allowed to cool and harden to form a tray. Next, the tray and the brackets (which are embedded in an interior wall of the tray) are detached from the stone model and sides of the tray are trimmed as may be desired. The tray also may be cut into smaller sections for ease of placement during bonding. If the cured adhesive remains on the bracket base after detaching the brackets from the model, the adhesive can serve as a custom-made bonding surface having a contour that matches the contour of the patient's tooth for a snug, mating fit.
Once the patient has returned to the office, a quantity of adhesive is placed on the base of each bracket (or on the cured adhesive, if any) and the tray (or tray section) with the embedded brackets is then placed over matching portions of the patient's dental arch. Since the configuration of the interior channel in the tray closely matches the respective portions of the patient's dental arch, each bracket is ultimately positioned on the patient's teeth at precisely the same location that corresponds to the previous location of the same bracket on the stone model.
Both light-curable adhesives and chemical curing adhesives have been used in indirect bonding techniques to secure the appliances to the patient's teeth. If a light-curable adhesive is used, the tray is preferably transparent or translucent. If a two-component chemical curing adhesive is used, the components can be mixed before application to the brackets, or alternatively one component may be placed on each bracket base (or on the cured adhesive, if any) and the other component may be placed on the tooth surface. In either case, placement of the tray with the embedded brackets on corresponding portions of the patient's dental arches enables the brackets to be bonded to the teeth as a group a relatively short amount of time. With such a technique, individual placement and positioning of each bracket in seriatim fashion on the teeth is avoided.
While it is apparent that the use of certain bonding techniques such as indirect bonding can greatly facilitate placement of appliances on the patient's teeth and shorten the amount of time that the patient is occupying the chair in the operatory, there is a continuing need in the art to improve current bonding techniques in order to increase placement accuracy, optimize the use of the practitioner's time and also improve the strength of the bond between the appliance and the tooth. Moreover, there is a need in the art to reduce the time and expenses associated with making the transfer tray, so that the expense to the practitioner as well as to the patient can be correspondingly decreased.